I cannot reproduce with latest F21 NetworkManager-0.9.10.1-1.git20150105.fc21. When NM starts up, it leaves virbr0 alone and shouldn't be starting DHCP on it. virbr0 has its own 192.168.122.1 address which NM leaves in place.
If you experience this issue still, could you grab:
I cannot reproduce with latest F21 NetworkManager- 0.9.10. 1-1.git20150105 .fc21. When NM starts up, it leaves virbr0 alone and shouldn't be starting DHCP on it. virbr0 has its own 192.168.122.1 address which NM leaves in place.
If you experience this issue still, could you grab:
journalctl -b -u NetworkManager
and attach that shows the issue?